While the advantages of obtaining a braking effect from the engine of a vehicle powered by an internal combustion engine are well known (see for example U.S. Pat. No. 3,220,392 to Cummins), an ideal braking system design characterized by low cost, simplicity, ease of maintenance and reliability has not yet been fully achieved. One well-known approach has been to convert the engine into a compressor by cutting off fuel flow and, opening the exhaust valve for each cylinder near the end of the compression stroke; thus, permitting the conversion of the kinetic inertial energy of the vehicle into compressed gas energy which may be released to atmosphere when the exhaust valves are partially opened. To operate the engine reliably as a compressor, rather exacting control is necessary over the timed relationship of exhaust valve opening and closing relative to the movement of the associated piston.
One technique for accomplishing this result is disclosed in U.S. Pat. No. 3,220,392 to Cummins, wherein a slave hydraulic piston opens an exhaust valve near the end of the compression stroke of an engine piston with which the exhaust valve is associated. The slave piston which opens the exhaust valve is actuated by a master piston hydraulically linked to the slave piston and mechanically actuated by an engine element which is displaced periodically in timed relationship with the compression stroke of the engine. One such engine element may be the exhaust valve train of another cylinder timed to open shortly before the first engine cylinder piston reaches the top dead center of its compression stroke. Other engine operating elements or components may be used to actuate the master piston of the braking system so long as the actuation of the master piston occurs at the proper moment near the end of the compression stroke of the piston whose associated exhaust valve is to be actuated by the slave piston. For example, certain types of compression ignition engines are equipped with fuel injector actuating mechanisms which are mechanically actuated near the end of the compression stroke of the engine piston with which the fuel injector train is associated, thus, providing an actuating mechanism immediately adjacent the valve which is to be opened. See also U.S. Pat. No. 3,405,699 to Laas.
The use of a hydraulically-linked master/slave piston assembly in a system for selectively converting an internal combustion engine from a power mode to a compressor or brake mode of operation has proven to be commercially viable and relatively simple especially in engines already equipped with appropriately timed fuel injector actuating mechanisms. However, certain difficulties have arisen during the operation of these braking systems. For example, the system disclosed in U.S. Pat. No. 3,220,392 uses a control valve which separates the braking system into a high pressure circuit and a low pressure circuit by using a check valve which prevents flow of high pressure fluid back into the low pressure supply circuit thereby allowing the formation of the hydraulic link in the high pressure circuit when in the braking mode. A three-way solenoid valve, positioned upstream of the control valve, controls the flow of low pressure fluid to the control valve and, thus, controls the beginning and end of the braking mode. When the engine is in a power mode distinct from the braking mode, the solenoid valve connects the low pressure circuit to drain which causes the control valve to fluidically connect the high pressure circuit to drain, thus, terminating-the hydraulic link. However, it has been found that under certain operating conditions, supply fluid having an undesirably high fluid pressure is supplied to the control valve via the solenoid valve during the braking mode. Such a high supply pressure has been found to cause the inadvertent movement of the slave piston and, thus, movement of the exhaust valves, inwardly beyond the design clearance limits between the valve face and the engine piston possibly causing damage to the valves, valve train assembly, piston/cylinder assembly and other engine components by, for example, contact between the engine piston and the exhaust valves. The uncontrollable high pressure surges in the low pressure supply circuit are often caused by an overly viscous supply of fluid. Fluid having an undesirably high viscosity may result from the thermal effects of low ambient temperature conditions on the fluid when the engine is not is use. Higher viscosity fluid causes an increase in the supply pressure from the supply pump into the low pressure circuit. Also, higher than expected supply pressures may result from a malfunction in the fluid supply pump feeding fluid to the low pressure circuit. Regardless of the cause, undesirably high fluid supply pressure in the low pressure circuit may cause serious damage to the engine when in the braking mode.
As a result, at least one attempt has been made to prevent such an occurrence. For example, as shown in FIGS. 5A-5C, the assignees of the present invention have designed a control valve having an integral pressure relief valve function to sense the fluid pressure in the low pressure circuit above a predetermined level and to respond by venting fluid from the high pressure circuit to prevent the pressure in the high pressure circuit from reaching a predetermined maximum pressure corresponding to the force needed to inadvertently move the exhaust valves into the cylinder of the engine. The control valve includes a slidably mounted control valve member including a spring biased check valve which prevents the flow from the high pressure circuit back into the low pressure circuit. The control valve member is spring biased into a first position by an inner spring thereby blocking flow through the control valve member and connecting the high pressure circuit to drain. When the solenoid valve is moved to an open position supplying fluid to the low pressure circuit to place the engine in a brake mode, the fluid pressure moves the control valve member compressing the inner spring until the control valve member contacts an outer spring thereby allowing fluidic communication between the high pressure and low pressure circuits via passages formed in the control valve member. In the event of an undesirably high supply pressure in the low pressure circuit, the control valve member moves further outwardly compressing the outer spring and connecting the high pressure circuit with a low pressure drain to reduce the pressure in the high pressure circuit to a predetermined level to prevent inadvertent and excessive movement of the slave piston. However, the braking system cannot function in the braking mode while the control valve is in the relief position venting fluid from the high pressure circuit since the control valve opens the high pressure circuit to drain, thus, disabling the hydraulic link which transmits the force from the master piston to the slave piston for moving the exhaust valves. As a result, the pressure relief function of the control valve of this design disadvantageously affects the reliability and the effectiveness of the braking system. Moreover, it has been found that this integral pressure relief valve/control valve design requires the inner spring of the control valve to experience excessive linear motion ultimately causing control valve spring failure. Design of a more appropriate and durable spring has been limited by the allowable package size of the control valve housing which does not permit for a spring design capable of withstanding the necessary displacements of the present control valve.
U.S. Pat. Nos. 4,150,640, 4,271,796 and 5,036,810 all disclose engine compression brake systems having a form of pressure relief means for relieving fluid pressure in the high pressure fluid circuit connecting the master and slave pistons. However, these systems do not allow continuous, uninterrupted operation of the system in the braking mode while the pressure relief means is functioning and, therefore, are not as reliable and effective as desired.